In order to perform electronic control of the variable valve timing control, the variable valve timing controller which has the motor as a source of the drive has been developed. The variable valve timing controller described in JP-2006-70754A (US2006/0042578A1) includes a first gear, a second gear, a phase changing gear, and an electric motor. The first gear (outer gear) is concentrically arranged with the camshaft and is rotated with the rotation driving force of the crankshaft. The second gear (inner gear) rotates together with the camshaft. The phase changing gear (planet gear) transmits the torque of the first gear to the second gear, and varies the rotational phase of the second gear relative to the first gear. The motor is coaxially provided to the camshaft so that the revolution speed of the phase changing gear is controlled. The number of teeth of the first gear, the second gear, and the phase changing gear is determined so that the camshaft may rotate with one half of the rotational speed of the rotational speed of the crankshaft.
In the above motor drive variable valve timing controller, as a driving current of the motor (“motor current”) increases during the variable valve timing control, the heat value of the motor increases and a coil temperature rises. When the transient operating condition in which a target motor speed (target valve timing) changes frequently continues, a coil temperature of the motor may exceed an allowable temperature and will cause durability deterioration and malfunction of the motor.
In order to solve the above matters, the present inventors has developed a variable valve timing controller which is able to restrict a motor current so that generated heat does not exceed a specified permissible value. In such a valve timing controller, a new diagnosis technology is needed.